Consumer interest in the use of ceramics has resulted in a demand for intricate large sanitary ware items such as toilet bowls, kitchen sinks, slabs, vanity, bowls and the like which are made of a ceramic material. Previously, a ceramic body made of vitreous china was used due to its property of being impermeable to water. However, the manufacture of intricate, large sanitary ware items using vitreous china is difficult since vitreous china has double the firing contraction rate of a fire clay body and also suffers from the problem of significant sagging during the firing of this material to a glassy state. Thus, when using vitreous china, a user has to increase the mould size to accommodate the higher contraction rate and has to reshape the mould to offset the complex distortions which take place during the firing process.
It would be desirable to provide a ceramic material which has low distortion and contraction properties compared to vitreous china. This would then allow a wider range of shaped products to be formed including the manufacture of a wider range of sanitary ware items.
Engobe compositions are liquid clay compositions which typically contain pottery stone, clays, kaolin and feldspar and are typically applied to the surface of a clay body. The purpose of the engobe can vary and includes providing color to the clay body, improving the surface texture of the clay body; and providing a base layer to add further ornamentation or patterns thereon.
A disadvantage with the known engobe composition is that the properties of thermal shock resistance, good glaze appearance and chemical resistance are poor for fired clay products and particularly for fired sanitary ware products.